Fracture initiation pressures in permeable poorly consolidated sands

Abstract

This paper studies the stress distribution around an injection well drilled through a layer of poorly consolidated sand. As a plastically strained zone may form around a hole drilled through this type of rock, both elastic and plastic theories must be applied. Solutions have been worked out in order to follow the development of stresses as the injection pressure increases from zero up to the point where a fracture is initiated. How these stresses develop is dependent on the rock and fluid properties, mainly Poisson's ratio and the ratio between the horizontal and vertical stress. Several different stress distributions will appear as the injection pressure is increased. Each stress distribution is composed of one or more different stress solutions linked together. This is solved by numerical iteration. The injection pressure is increased until a fracture is induced. Fracture initiation pressures for several different parameter sets have been examined. The results have then been compared with fracture initiation pressures calculated by classical elastic theory. This comparison shows that the fracture initiation pressures can be lower than results calculated by elastic theory. Typically, with a Poisson's ratio of 0.20 and a horizontal to vertical effective stress ratio of 0.40, the difference is in the order of magnitude of 10%.